Bruton's tyrosine kinase (Btk), a member of the Tec family of non-receptor tyrosine kinases, plays an essential role in the B-cell signaling pathway linking cell surface B-cell receptor (BCR) stimulation to downstream intracellular responses. It is required for the normal development and function of B-lymphocytes in humans and mice as evidenced by mutations in the Btk gene that result in the X-linked agammaglobulinemia (XLA) phenotype in humans and a less severe X-linked immunodeficiency phenotype (xid) in mice. See, e.g., D. A. Fruman, et al., (2000), Immunity 13:1-3. Btk is expressed in all hematopoietic cells types except T lymphocytes and natural killer cells, and participates in a number of TLR and cytokine receptor signaling pathways including lipopolysaccharide (LPS) induced TNF-α production in macrophages, suggesting a general role for Btk in immune regulation.
Btk contains an amino-terminal pleckstrin homology (PH) domain, followed by a Tec homology (TH) domain, regulatory Src homology (SH3, SH2) domains, and a C-terminal kinase (SH1) domain. In unstimulated B cells, Btk is localized to the cytoplasm where it is catalytically inactive, presumably due to a tertiary conformation arising from intramolecular interactions between the kinase domain and the SH2 and/or SH3 domains that block access of substrates to the active site. After BCR stimulation, Btk is recruited to the cell membrane via interactions between the N-terminal PH domain and cell membrane phosphoinositides. Membrane-associated Btk is then phosphorylated at Tyr 551 in the activation loop by Src family kinases. Subsequent Btk auto-phosphorylation at Tyr 223 stabilizes the active conformation and fully activates Btk kinase activity. Activated Btk phosphorylates phospholipase (PLCγ), initiating calcium mobilization and generating diacylglycerol (DAG) as secondary signals, eventually leading to transcriptional activation and amplification of BCR stimulation.